Phototransformation Of Benzophenone UV Filters In Aquatic Environment

Benzophenone UV filters（BPs）are widely used in personal care products,such as sunscreens and cosmetics.They are released to the aquatic environment through water recreational activities and discharge from wastewater treatment plants.BPs can disrupt endocrine systems of animals,have genotoxicity and other hazards.They have raised great concern due to their potential ecological risks.Nitrite（NO₂^-）is an important photosensitizer in surface waters.It can decompose to form hydroxyl radical（·OH）,nitrogen dioxide（NO₂^·）and other reactive species upon UV irradiation.In this paper,photochemical degradation of four BPs,i.e.,2,4-dihydroxybenzophenone（BP1）,2-hydroxy-4-methoxy benzophenone（BP3）,2-hydroxy-4-methoxy-5-sulfonate benzophenone（BP4）and 2,2’-dihydroxy-4-methoxy benzophenone（BP8）in the presence of nitrite were comprehensively studied.BPs per se are photochemically stable.Under the irradiation of UV_356nm,no appreciable attenuation was obversed in 2 h,except BP1 reduced by 12%.However,their degradation was greatly promoted by the presence of nitrite.The rates of BPs degradation in the UV/nitrite process were apparently first order to their own concentrations.Degradation of BPs was due to the reactions with·OH and NO₂^·generated by nitrite photolysis.The bimolecular rate constants of BP1,BP3,BP4,and BP8 reacting with·OH were measured to be 1.1×10¹⁰,9.2×10⁹,7.5×10⁹,and 8.1×10⁹M^-1s^-1,respectively,using competition kinetic approach.The bimolecular rate constants of each individual BP with NO₂^·were calculated to be 5.6×10⁴,3.6×10⁴,1.0×10²,and2.8×10⁴ M^-1 s^-1,respectively,by kinetic modeling.Kinetic modeling also revealed that the steady-state concentrations of NO₂^·were approximately 6 orders of magnitude higher than that of·OH in the UV/nitrite process,although the bimolecular rate constants of its reactions with BPs were 6 orders of magnitude lower.With the increase of nitrite concentration,BPs degradation was accelerated,and the contribution of NO₂^·increased as well.At initial nitrite concentration of 10μM,and the contributions of·OH and NO₂^·to BP1 degradation were 66.1%and 21.5%,respectively.However,NO₂^·only contributed a tiny fraction of the degradation of BP4,due to the presence of an electron-withdrawing sulfonate group in the molecule.·OH and NO₂^·reacting with BPs formed hydroxylated and nitrated products,respectively.Natural organic matter（NOM）inhibited the nitrite sensitized degradation of BPs,due to the light screening effect and radical scavenging effects.Among the four BPs,BP1 was relatively susceptible to UV_365nm radiation.Approximately 12%removal occurred in 2 h.With prolonged irradiation time,its degradation was accelerated and almost completed removal was achieved in the following 2 h.Excited triplet state and singlet oxygen（¹O₂）quenching experiments confirmed that ¹O₂ was responsible for the degradation of BP1.¹O₂was initially generated by the excited triplet state of BP1 through energy transfer to the dissolved oxygen in air-saturated water.It is noted that BP1 reacted with ¹O₂ producing 2-[（2E）-1-oxo-1,2-dihydronaphthalen-2-ylidene]acetic acid and other products.These products all have a benzophenone structure and have higher photochemical activities than that of the parent compound.Once formed,they could be excited and sensitize the dissolved oxygen to generate more ¹O₂,and ¹O₂ continued to react with BP1 to generate more active products.As a result,the degradation of BP1 was accelerated.In summary,this study revealed that nitrite at environmentally relevant levels could greatly promote the degradation of BPs.However,nitrite-sensitized photo transformation of BPs would lead to nitrated products which are usually toxic to aquatic organisms and recalcitrant to oxidative degradation.In addition,BP1 undergoes self-sensitization photolysis under UV_365nm irradiation,producing products with higher photochemical activity.BP1 may cause skin damage when exposed to the sun for long as sunscreen ingredients.